The invention relates to a method of transmitting data from a plurality of mobile sub-stations to a stationary main station by way of a common channel which is subdivided into time slots the length synchronization of the substations being determined by a corresponding time slot distribution of a data transmission signal transmitted by the main station. Data packets have a time length essentially equal to the time length t.sub.S of the time slots and are transmitted by the sub-station slightly earlier than the beginning of the relevant time slot. The "advancement of time" is selected in such a way that the data packets, when received by the main station, are substantially synchronized with the time slot distribution of the main station.
A method of this type is used, for example, in a vehicle communication system. It may then relate to a communication system arranged for data exchange among vehicles, in which the communication is effected by way of the main station then acting as an information relay station. The communication system may also be arranged, for example, as a traffic navigation system, in which the main station provides the vehicles with traffic information.
The communication path from the main station to the sub-stations will henceforth be called the downlink, whereas the communication path from the sub-stations to the main station will henceforth be called the uplink. The carrier frequency used for the downlink differs from the carrier frequency used for the uplink and is continuously monitored by all the active sub-stations.
A problem for such a communication system relates to the fact that the distance from each mobile substation to the main station is variable, as a result of which the degree of synchronization of the uplink time slot distribution relative to the downlink time slot distribution may decrease. Consequently, messages may get lost, or termed differently, the user efficiency of the communication channel (denoted by the English term of throughput and expressed in the number of data bits received correctly via a communication channel per second divided by the maximum number of data bits that can be transmitted per second via that communication channel) may be reduced.
U.S. Pat. No. 4,472,802 has disclosed solving this problem by permitting the main station to determine the time delay with which a message sent by a sub-station is received, and inform that sub-station of this time delay so that the sub-station sends its messages with a time delay and so that the messages are received within one time slot by the main station. However, this known principle has a drawback of at least three time slot periods getting lost before the messages are sent with the desired advancement of time by the sub-station.